Muscular dystrophies (MDs) are a broad group of inherited diseases that are characterized by progressive skeletal muscle weakness and wasting with a variable clinical course. The molecular mechanisms whereby individual skeletal muscle fibers undergo degeneration have not been well defined, and there are still no cures for these diseases. This proposal explores the therapeutic potential of mouse embryonic stem (ES) cells, with the ultimate goal of discovering principles that govern, the differentiation of ES cells into myogenic precursor cells that can be used in cell transplantation in murine models of MDs. To date, there has been no definitive demonstration that stem cells that arise in vitro during differentiation into embryoid bodies (EBs) are capable of engrafting either normal or dystrophic adult muscle. In collaboration with Cossu and colleagues, the Campbell laboratory showed that mesoangioblasts, stem cells isolated from embryonic dorsal aorta, can correct the phenotype in mouse model of MD. We are thus well positioned to investigate the nature of the common vascular-myogenic progenitor that can be formed in EBs, and the mechanisms governing the potential for such cells to engraft in adult myogenic microenvironments. We are proposing methods for isolating enriched populations of EB-derived progenitors using either selectable markers driven by myogenic specific promoters or cell purification via surface markers, and will endeavor to more carefully define the surface antigen phenotype and in vivo properties of this population of cells. We will test the hypotheses that engraftment is facilitated by homing, enhanced cell survival, or cell proliferation, and probe the molecular basis for these observations using a non-invasive bioluminescence monitoring system that will also allow us to investigate the long-term effect of cell-mediated therapy. Our results will enable methods for enhanced myogenic development from ES cells and reconstitution of the adult myogenic system as a model for myogenic research and cellular therapies. Future efforts will explore the similarities and differences in the myogenic potential of murine and human ES cells, and the therapeutic potential of in vitro differentiated human ES cells.